Accepted Manuscript Title: Ag2 C2 O4 and Ag2 C2 O4 /TiO2 Nanocomposites as Highly Efficient and Stable Photocatalyst under Visible Light: Preparation, Characterization and Photocatalytic Mechanism Authors: Caixia Feng, Mingming Sun, Yan Wang, Xianshun Huang, Anchao Zhang, Yuhua Pang, Yanmei Zhou, Linshan Peng, Yangting Ding, Ling Zhang, Deliang Li PII: DOI: Reference:
S0926-3373(17)30730-0 http://dx.doi.org/doi:10.1016/j.apcatb.2017.07.081 APCATB 15913
To appear in:
Applied Catalysis B: Environmental
Received date: Revised date: Accepted date:
31-5-2017 23-6-2017 26-7-2017
Please cite this article as: Caixia Feng, Mingming Sun, Yan Wang, Xianshun Huang, Anchao Zhang, Yuhua Pang, Yanmei Zhou, Linshan Peng, Yangting Ding, Ling Zhang, Deliang Li, Ag2C2O4 and Ag2C2O4/TiO2 Nanocomposites as Highly Efficient and Stable Photocatalyst under Visible Light: Preparation, Characterization and Photocatalytic Mechanism, Applied Catalysis B, Environmentalhttp://dx.doi.org/10.1016/j.apcatb.2017.07.081 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Brief Summary
Ag2C2O4 and Ag2C2O4/TiO2 Nanocomposites were prepared via liquid precipitation method. Both of them were capable of photocatalytic degradation of propylene and acetaldehyde under visible light irradiation. The mechanism of highly efficient and stable photocatalytic activity was proposed. On the one hand, Ag2C2O4 undergoes firstly photo-corrosion during the initial photocatalytic process generating nascent metallic Ag on the surface of Ag2C2O4, which subsequently endowing the Ag@Ag2C2O4 capable of visible light phtocatalytic activity due to the plasma resonance of Ag. On the other hand, Ag2C2O4/TiO2 nanocomposites also experience the same journey resulting in the Ag@Ag2C2O4/TiO2. Differently from Ag@Ag2C2O4, two paths are involved to separate the electron and hole excited by the photon of nascent metallic Ag so as to acquire much higher phtocatalytic activity for Ag@Ag2C2O4/TiO2.
Ag2C2O4 and Ag2C2O4/TiO2 Nanocomposites as Highly Efficient and Stable Photocatalyst under Visible Light: Preparation, Characterization and Photocatalytic Mechanism Caixia Feng a, Mingming Sun a, Yan Wang a, c, *, Xianshun Huang a, Anchao Zhang b, *, Yuhua Pang a, Yanmei Zhou a, Linshan Peng a, Yangting Ding a, Ling Zhang a, Deliang Li a a
Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical
Engineering, Henan University, Kaifeng 475004, China b
School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo, 454000,
China c
Department of Scientific Research, Henan University, Kaifeng 475004, China
* Corresponding Author: Yan Wang, E-mail:
[email protected] Anchao Zhang, E-mail:
[email protected];
Graphical abstract
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Research Highlights
► Ag2C2O4 and Ag2C2O4/TiO2 nanocomposites as highly efficient and stable photocatalyst were prepared. ►Both Ag2C2O4 and Ag2C2O4/TiO2 undergo photo-corrosion but still show high
photocatalytic activity. ►The visible light activity of Ag2C2O4/TiO2 was much higher than that of pure Ag2C2O4 and P25-TiO2. ►Different photocatalytic mechanisms for Ag2C2O4 and Ag2C2O4/TiO2 were proposed, respectively.
ABSTRACT Ag2C2O4 and Ag2C2O4/TiO2 (Commercial P25-TiO2) nocomposites with different weight ratios of Ag2C2O4 to P25-TiO2 (denoted as Ag2C2O4/TiO2-0.02, Ag2C2O4/TiO2-0.08, Ag2C2O4/TiO2-0.15, Ag2C2O4/TiO2-0.25, Ag2C2O4/TiO2-0.3) were synthesized by a facile liquid precipitation method. The photocatalytic activity of samples was evaluated by the degradation of propylene and acetaldehyde gas under visible light irradiation. All the samples displayed a highly efficient and stable photocatalytic activity. Furthermore, compared to pure Ag2C2O4 and P25-TiO2 (used as reference), Ag2C2O4/TiO2 nanocomposites showed much higher photocatalytic activity and Ag2C2O4/TiO2-0.25 possessed the highest photocatalytic activity. The kinetic behavior of Ag2C2O4/TiO2-0.25 was well accordant with a pseudo-first-order reaction and the reaction rate constants were calculated to be 11.13 h-1 and 0.5376 h-1 for the degradation of propylene and acetaldehyde, respectively. The mechanism of highly efficient and stable photocatalytic activity was discussed. On the one hand, Ag2C2O4 undergoes firstly photo-corrosion during the initial
photocatalytic process generating nascent metallic Ag on the surface of Ag2C2O4, which subsequently endowing the Ag@Ag2C2O4 with visible light phtocatalytic activity due to the plasma resonance of Ag. On the other hand, Ag2C2O4/TiO2 nanocomposites also experience the same journey resulting in the formation of Ag@Ag2C2O4/TiO2. Differently from Ag@Ag2C2O4, two paths are involved to separate the electron and hole excited by the photon of nascent metallic Ag so as to acquire much higher phtocatalytic activity for Ag@Ag2C2O4/TiO2.
Keywords: Ag2C2O4/TiO2; photo-corrosion; visible light; plasma resonance; photocatalytic activity 1. Introduction In recent years, many silver-containing photocatalytic materials have received great attention due to their proper band gap and relatively good performance in degradation of pollutants
1-7
. In
2008, Huang and coworkers synthesized Ag@AgCl as highly efficient and stable photocatalyst capable of photooxidation of methylic orange under visible light. They attributed the photocatalytic activity to the plasmon resonance of silver deposited on AgCl particles 1. Ye and coworkers reported the use of Ag3PO4 as an active visible-light-driven photocatalyst for the oxidation of water and photodecomposition of organic compounds. It was proposed that the p-block elements are actually important ingredients for high photocatalytic activity 4. Adhering to this idea, Huang and coworkers prepared silver carbonate (Ag2CO3) short rods using a precipitation method. The prepared Ag2CO3 displayed a high activity towards degradation of phenol and methylene blue under visible light 5. Dong and coworkers also reported the high-efficiency visible-light sensitive Ag2CO3 photocatalyst prepared by a simple ion-exchange
method based on a strategy incorporating of p-block C element into a narrow bandgap Ag2O. The theory calculation based on the plane-wave-based DFT indicates the incorporation of p-block C element into narrow bandgap Ag2O broadens the bandgap width and enhances photodegradation ability 6. However, these Ag-based photocatalysts usually experience photo-corrosion under visible light irradiation, which causes damage to their photocatalytic activity and thus, their practical applications are limited. To improve the photocatalytic activity and stability of silver-containing catalysts, many attempts have been performed to prevent the photo-corrosion and enhance the photocatalytic activity. One way is to add sacrificial agent as an electron acceptor in the reaction system
4,5,8
. Dai and coworkers investigated the photo-corrosion mechanism of Ag2CO3 in
aqueous solution and proposed that the photocorrosion of Ag2CO3 can be efficiently inhibited by adding AgNO3 in the photocatalytic reaction system owing to the lower electrode potential of Ag/AgNO3 than that of Ag/ Ag2CO3 8. Another widely used way is coupling the Ag-based photocatalysts with each other or other semiconductor materials to fabricate heterostructured photocatalysts
9-24
. On the one hand, Yu and coworkers coupled AgI into Ag2CO3, generating
novel AgI/Ag2CO3 composite photocatalyst. The formation of AgI/Ag2CO3 heterojunction with intimate interface could effectively increase the separation efficiency of the e−/h+ pairs and promote the production of •OH and O2•− radicals, which is beneficial to the photocatalytic degradation of MO 19. Graphene interfaced with Ag3PO4, Ag3VO4, or Ag2CO3 were systematically studied and graphene was proposed as an excellent electron acceptor and transporter, which markedly promoted the separation of photoexcited electron–hole pairs
15,17,18
. On the other hand,
TiO2 sensitized by Ag-containing photocatalysts were achieved to enhance the stability and
photocatalytic efficiency. Rawal and coworkers synthesized Ag3PO4/TiO2 by covering the surface of Ag3PO4 with polycrystalline TiO2 via sol–gel method. The remarkable photocatalytic activity of Ag3PO4/TiO2 was attributed to the inter-semiconductor hole-transfer between the valence band of Ag3PO4 and TiO2 12. Yu and coworkers found that coupling of trace Ag2CO3 with TiO2 could enrich the surface OH groups of the sample and suppress the recombination rate of photogenerated electrons (e−) and holes (h+) pairs
13
. Mohaghegh and coworkers reported that
Ag2CO3 sensitized TiO2 prepared in ionic liquid medium formed the Ag2CO3/TiO2/RTIL (2-hydroxylethylammonium formate) heterostructure affording to the promoted absorption of light and more efficient suppression of charge carrier recombination
14
. In our previous work,
anatase TiO2 containing a large amount of single-electron-trapped oxygen vacancy (denoted as TiO2(Vo•); Vo• refers to single-electron-trapped oxygen vacancies which are abridged as SETOVs) or P25-TiO2 was combined with Ag2CO3 to prepare Ag2CO3/TiO2(Vo•) or Ag2CO3/P25 by the precipitation method
25-27
. The synergistic effect between the oxygen vacancies and nascent
metallic Ag accounts for the high and stable photocatalytic activity of Ag2CO3/TiO2(Vo•) 25, while the synergetic effect between surface plasma resonance of silver and interfacial electron transfer is in favor of the enhanced photocatalytic performance of Ag2CO3/P25
27
. In reference [26], we
accidentally found that wide band gap semiconductor Ag2C2O4 also showed highly efficient and stable visible light photocatalytic activity. As far as we know, no one has been reported that Ag2C2O4 could be used as visible-light-active photocatalyst and its physicochemical properties, photo-corrosion, visible light photocatalytic behavior as well as photocatalytic mechanism should be further systematically studied. In the present work, pure Ag2C2O4 and a series of Ag2C2O4/TiO2 heterostructures with different
weight ratios of Ag2C2O4 to P25-TiO2 were prepared by a facial precipitation method. The resulting Ag2C2O4 and Ag2C2O4/TiO2 nanocomposite was characterized by means of scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS). The comparison of XPS and XRD results of Ag2C2O4 and Ag2C2O4/TiO2 between before and after irradiation suggested that both pure Ag2C2O4 and ingredient Ag2C2O4 of Ag2C2O4/TiO2 endured photo-corrosion to some extents generating nascent metallic Ag. As well known, such a photo-corrosion is originally non-conducive to the photocatalysts. However, both Ag2C2O4 and Ag2C2O4/TiO2 nanocomposite showed highly efficient and stable photocatalytic activity toward the degradation of propylene and acetaldehyde gas under visible light. Compared to pure Ag2C2O4 and P25-TiO2, Ag2C2O4/TiO2 heterostructures noticeably exhibited an improvement in both light absorption and photoactivity. The photocatalytic mechanisms of Ag2C2O4 and Ag2C2O4/TiO2 were discussed in detail and proposed in this paper. 2. Experimental All chemicals are of analytical grade purity and were used without further purification. 2.1 Catalysts preparation Ag2C2O4 was chemically synthesized by direct reaction in the aqueous solutions between AgNO3 and Na2C2O4 as the starting materials. Briefly, 5 mL of Na2C2O4 (0.1 M) was added into a 50-mL beaker. Into Na2C2O4 solution was then slowly added 15 mL of AgNO3 (0.1 M) under 20 min of magnetic stirring to afford white precipitate Ag2C2O4. As-obtained Ag2C2O4 was filtered, washed with distilled water, and dried at 60 ◦C in dark. Ag2C2O4/TiO2 nanocomposite photocatalysts were synthesized by an in-situ deposition of
Ag2C2O4 onto the surface of P25-TiO2. Briefly, 0.5g of P25-TiO2 powder was dispersed in distilled water and mixed with a quantity of Na2C2O4 solution (0.05 M) under 10 min of magnetic stirring to afford a uniform suspension. Into the above mixed suspension was dripped an equal volume of AgNO3 (0.1 M) solution under 20 min of magnetic stirring to yield the precipitate. The resultant precipitate was filtered, washed with distilled water, and dried at 60◦C in dark in an oven to provide disired Ag2C2O4/TiO2 nanocomposite. The produced Ag2C2O4/TiO2 nanocomposites with different theoretical weight ratios (Ag2C2O4:P25) of 0.02:1, 0.08:1, 0.15:1, 0.25:1, 0.3:1 were named as Ag2C2O4/TiO2-0.02, Ag2C2O4/TiO2-0.08, Ag2C2O4/TiO2-0.15, Ag2C2O4/TiO2-0.25, Ag2C2O4/TiO2-0.3, respectively.. 2.2 Characterization The morphology of as-prepared photocatalysts was examined using field emission scanning electron microscopy (SEM, JEOL JSM-7610F) and transmission electron microscopy (TEM, JEM-2010). The crystal phase compositions of samples were measured with a Bruker D8 X-ray diffractometer (XRD) with Cu Kα radiation at an accelerating voltage of 40 kV. The ultraviolet-visible light diffusion reflectance spectra (DRS) of the photocatalysts were recorded with a PE Lambda 950 spectrometer equipped with an integrating sphere attachment (BaSO4 was used as a reference). X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos AXIS system with a monochromatic Al-Kα source (hν = 1486.6 eV). The binding energies were calibrated with reference to adventitious C 1s line at 284.8 eV. The photocurrent was carried out by electrochemical station (CHI 650E Chenhua Instrument Company) in a three-electrode quartz cell with 0.1 mol L−1 Na2SO4 electrolyte solution.
A
xenon lamp equipped with an ultraviolet cutoff filter was served as the visible light source (> 420
nm; light intensity: 1 mW cm−2). A platinum wire and Ag/AgCl were used as counter electrode and reference electrode, respectively. The working electrode was prepared as follows: 2 mg of sample, 30μL of water and 5 μL of Nafion were mixed to form homogeneous slurry. The resultant slurry was dropped onto the precleaned ITO glass and dried at room temperature for 12 h. 2.3 Evaluation of visible light photocatalytic activity The experiment of photo-degradation of propylene was performed in an on-line analysis system (denoted as system I) equipped with a gas chromatograph (GC7900) to monitor the concentration change of C3H6 (C), which has been reported elsewhere in our previous work
28
. In system I, 25
mg aliquot of each sample was spread on one side of a roughened glass plate (ca. 10cm2) located in a quartz tube reactor equipped with a 300 W xenon lamp as the visible light source. Between the xenon lamp and reactor was inserted an ultraviolet (UV) cut 420 filter to eliminate UV light. The reactor was surrounded by a water channel so as to eliminate infrared light and keep a constant reaction temperature at room temperature. The feed gas was mixed of propylene and dry air, and was allowed to flow through the reactor at a flow rate of 200 mL/h. Prior to irradiation, the feed gas was allowed to flow through the reactor continuously until the adsorption/desorption equilibrium was established. The concentrations of C3H6 and CO2 production were measured by GC equipped with a flame ionization detector (FID), a GDX-502 column, and a reactor loaded with Ni catalyst for methanization of CO2. The removal rate of C3H6 is calculated as (C0 − C)/C0 × 100%, where C0 refers to the initial C3H6 concentration with a value of 500 ppmV. The experiment of photo-degradation of acetaldehyde gas was performed in a steady state photocatalytic system (denoted as system II). In system II, as-evaluated catalysts was uniformly spread on the bottom of a quartz cylindrical reactor with a build-in port for collecting the inside
feed gas. The feed gas is made up of acetaldehyde gas and dry air with a theoretically-calculated acetaldehyde gas concentration of 800 ppmV. The same optical source kept with a 25 cm distance above to the as-evaluated catalysts was used as the visible light source. At given intervals of irradiation, 1 mL of the feed gas was drawn off from the build-in port and immediately injected into the gas chromatograph to measure the concentration of acetaldehyde gas. 3. Results and discussion 3.1. Morphology and structure The SEM and TEM images of Ag2C2O4, P25-TiO2 and Ag2C2O4/TiO2 nanocomposites are shown in Fig 1. It can be seen from Fig. 1a that the size of Ag2C2O4 is unsurprisingly located in micron scale fluctuating from 0.5 to 3 μm due to the simple precipitation method and the surface is relatively smooth. It is well known that P25-TiO2 samples are consist of nanoparticles with a diameter of 20 ~ 30 nm, which is well in accordance with the results shown in Fig. 1b. As for Ag2C2O4/TiO2 nanocomposites, the morphology of P25-TiO2 shown in Fig. 1c is almost unchanged. Surprisingly, one could see that superfine Ag2C2O4 with the size of 1 ~ 2 nm was well formed and dispersed on the surface of P25-TiO2. Compared with the synthetic process of pure Ag2C2O4, some chemical interaction must have been occurred between P25-TiO2 with Na2C2O4 before dripping into AgNO3 solution. It is well known that there are valance-unfilled Ti(IV) ion centers and O(II) centers which form basic ≡TiOH and acidic ≡OH on the P25-TiO2 particle surface in aqueous solution, respectively interact with basic ≡TiOH as follows:
29
. Therefore, it could be referred that oxalate ion may
When AgNO3 solution was dripped into above mixture, oxalate ion would be robbed by Ag+ and gradually released to generate superfine Ag2C2O4 due to the slow-release effect. Taking into the value of solubility product constant (Ksp(Ag2C2O4) = 3.5×10-11 << Ksp(AgOH) = 2.0×10-8), it can be confirmed that the product was Ag2C2O4 rather than AgOH or Ag2O (transformed from AgOH). Fig. 2 shows the XRD patterns of pure Ag2C2O4 and Ag2C2O4/TiO2 nanocomposites with different weight ratios of TiO2 to Ag2C2O4. On the one hand, it can be seen from Fig. 2 (curve f) that the pure Ag2C2O4 displayed obvious diffraction peaks at 2θ = 17.15°, 19.21°, 29.94° and 32.38°, which can be indexed to the crystallite planes of (110), (200), (011) and (111) in monoclinic phase of Ag2C2O4 (JCPDS No: 22-1335). On the other hand, P25-TiO2 is well known composed of anatase TiO2 as the major phase and rutile TiO2 as the minor phase (not given here) 30
. After Ag2C2O4 was incorporated into P25-TiO2, the Ag2C2O4/TiO2 nanocomposites displayed a
miscible structure covering monoclinic phase Ag2C2O4 mixed with anatase phase as well as rutile phase of P25-TiO2, indicating that Ag2C2O4 have been deposited on the surface of P25-TiO2 (see curves b ~ e). Furthermore, with the increase of the ratio of Ag2C2O4 to TiO2 from 0.02:1 to 0.3:1, the intensity of monoclinic diffraction peaks of (110), (200), (011) and (111) improved remarkably and the others diffraction peaks gradually appeared. 3.2. XPS analysis The X-ray photoelectron spectroscopy (XPS) was carried out to further investigate the composition and the chemical status of samples. Fig. 3A and B demonstrate the C 1s and Ag 3d XPS spectra of both Ag2C2O4 and Ag2C2O4/TiO2-0.25 nanocomposite (possess the best photocatalytic performance), respectively. For both samples, the high resolution C 1s spectra
display two peaks locating at 284.8 and 288.4 eV (see Fig. 3A), respectively. The former main peak can be assigned to carbon contamination from XPS instrument, while the latter peak is derived from the carbon element in Ag2C2O4 27. Fig. 3B exhibits the narrow scan spectrum of Ag 3d at the binding energy of 367.8 eV (Ag 3d5/2) and 373.9 eV (Ag 3d3/2) with an energy splitting of ca. 6.1 eV, which are well consistent with the values reported for Ag+ ions 13,19,31, excluding the presence of any Ag0 species in both fresh samples. 3.3. Optical property analysis The UV-visible absorption spectra of Ag2C2O4, P25-TiO2, and Ag2C2O4/TiO2 nanocomposites are given in Fig. 4. One could see that both pure Ag2C2O4 and P25-TiO2 only absorb ultraviolet light in association with the absorption-edge locating at about 324 nm and 390 nm, respectively. The absorption edge of the sample can be determined from the following equation: Eg = 1239.8 / λ
(1)
Where Eg is the band gap (eV) and λ (nm) is the optical absorption edge. Namely, the band gap energies of Ag2C2O4 and P25-TiO2 could be calculated for 3.83 eV and 3.18 eV, respectively, both of which belong to wide band gap semiconductors. The valence band (VB) edge position of Ag2C2O4 was estimated according to the concept of electronegativity
8,32,33
. The conduction band (CB) and VB potentials of Ag2C2O4 at the point of
zero charge can be calculated by the following equation: EVB = X – Ee + 0.5Eg
(2)
Where EVB is the valence band edge potential; Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV); and Eg is the band gap of the Ag2C2O4 that have been calculated as 3.83 eV; X is the absolute electronegativity of the semiconductor, which is defined as the geometric mean of
the absolute electronegativity of the constituent atoms by the following equation 34: X = (x1r∙ x2s∙ ∙ ∙ xn-1p ∙ xnq)1/N
(3)
where xn is the electronegativity of the constituent atom; n is the number of the species; N is the total number of atoms in the compound; The superscripts r, s, p, ... , q refer to the numbers of the atoms 1, 2, ... , n-1 and n, respectively in the molecule, so that r + s +∙∙∙∙∙∙+ p + q = N. The x values of Ag, C and O are 4.44, 6.27 and 7.54 eV, respectively 35. According to equation 2 and 3, the values of X and EVB for Ag2C2O4 are calculated to be 6.31 and 3.72 eV. The conduction band position can be determined to be -0.11 eV by ECB = EVB - Eg. In case of Ag2C2O4/TiO2 nanocomposites, the absorption extends from UV to visible light region and shows stronger absorption around 450 ~ 650 nm. The inset in Fig. 4 displays the absorptivity at 500 nm as a function of the weight ratio of Ag2C2O4 to TiO2. It initially increases with increasing the weight ratio and then reaches a nearly steady value. Considering the preparation and DRS measurement method, two factors can be ascribed to the visible light absorption. One is that there probably formed Ag2C2O4/TiO2 heterojunction resulting in quantum effect beneficial to the visible light absorption. Another probably reason could be ascribed to the surface plasmon resonance effect of Ag nanoparticles that derived from mechanical grinding in quartz bowl before DRS measurement. Some reporters have been found that Ag oxalate could be decomposed into Ag nanoparticles by mechanochemical method even simple collisions
36-38
. Therefore, it can be
inferred that a handful of Ag formed on the surface of Ag2C2O4/TiO2 nanocomposites due to the grinding effect. Moreover, the characteristic absorption of the surface plasmon resonance effect of Ag nanoparticles is just lying in the range of 450 ~ 700 results.
39-41
, well accordance with the above
3.4. Transient photocurrent response Transient photocurrent responses were determined to investigate the charge separation and migration of photocatalysts. Fig. 5 demonstrates the comparison of photocurrent responses of different samples conducted under intermittent visible light irradiation. There were no photocurrent responses for P25-TiO2 possibly due to the wide band gap. As for Ag2C2O4/TiO2 nanocomposites, one could see that the photocurrent rises rapidly to a constant value when the light was turned on, while the photocurrent immediately decreases to nearly zero as long as the light was off. It can be seen from Fig. 5 that photocurrent density were reproducible during the repeated on–off cycles under visible light irradiation, indicating photocatalysts were stable under the illumination
42
. There appears an anodic photocurrent peak at the initial time of irradiation
followed gradually decay until light-off. When the light is switched off, a cathodic peak can also be observed. The initial current can be attributed to the separation of electron-hole pairs at the photocatalysts/electrolyte interface. The decay of the photocurrent indicates that a fraction of the holes reaching the semiconductor surface, instead of capturing electrons from the electrolyte, either recombine with electrons from the conduction band and/or accumulate at the surface
33
.
When the light is switched off, the holes accumulated in the surface still continue to recombine, resulting in the cathodic peak. The above results indicates that there may be exist some grain boundaries in the direction of electron diffusion Ag2C2O4/TiO2 heterojunction
9,13,14,24
43
, very possibly originated from the
. Ag2C2O4/TiO2-0.02 showed nearly no photocurrent
responses, which can be ascribed to the trace Ag2C2O4 incorporation well in accordance to the optical absorption results. With the increase of Ag2C2O4 weight ratio, the photocurrent density initially increases from 0.15 mA•cm-2 for Ag2C2O4/TiO2-0.08 to a maximum value of 1.38 mA•
cm-2 for Ag2C2O4/TiO2-0.25, respectively. Further increase in the Ag2C2O4 weight ratio up to 0.3 leads to a decrease in photocurrent density from 1.38 mA • cm-2 to 0.85 mA • cm-2 (for Ag2C2O4/TiO2-0.30). Moreover, the photocurrent of Ag2C2O4/TiO2-0.3 was terribly unstable, indicating that more Ag2C2O4 incorporated into P25-TiO2 was not benefit to the charge separation and migration, well accordance with its photocatalytic performance. 3.5 visible light photocatalytic performance The experiment of propylene degradation was measured in system I, equipped with a gas chromatograph for in-situ analysis. Prior to irradiation, the feed gas was flowing continuously until the propylene adsorption-desorption equilibrium was established. Figure 6 shows the photocatalytic degradation of propylene over P25-TiO2, Ag2C2O4 and Ag2C2O4/TiO2 nanocomposites and CO2 production under visible light irradiation. The propylene removal rate over P25-TiO2 is only 8% (see Fig. 6A) under visible light irradiation, corresponding to inertness of P25-TiO2 with a large band gap towards visible light photocatalytic oxidation of propylene. Contrary to the above, propylene is photodegraded as soon as the light is turned on, and the maximum removal rate of propylene over Ag2C2O4 is up to ca. 52.7%, even Ag2C2O4 possessing wider band gap than P25-TiO2. Unfortunately, the visible light photocatalytic activity of Ag2C2O4 gradually declines with extending reaction time. This indicates that Ag2C2O4 is unstable possibly due to photo-corrosion under visible light irradiation, which can be confirmed by the CO2 production results. As can be seen from Fig. 6B, a CO2 concentration spike with a value of 34711 ppmV, which immediately declined continuously, appears at the initial time of irradiation over Ag2C2O4. Noticing that 3 mol CO2 should be generated by complete oxidation of 1 mol of C3H6, the obtained CO2 concentration is far more than that of complete oxidation of propylene (500 ppm
× 52.7% × 3 = 795 ppm). It can be inferred that there must have other sources of CO2. Considering that Ag-based photocatalysts usually experience photo-corrosion under visible light irradiation, the extra more CO2 content should be derived from the photo-corrosion of Ag2C2O4 as follow: Ag2C2O4 + hν → 2Ag + 2CO2
(4)
With extending the irradiation time, the nascent metallic Ag generated on the surface of substrate Ag2C2O4 should hinder the photo-corrosion happening. Therefore, the gradual decay of CO2 concentration was observed and a plasmonic photocatalyst Ag@Ag2C2O4 formed thereby 44, accounting for the unstable visible light photocatalytic performance. Differently from the instability of Ag2C2O4, all the resultant Ag2C2O4/TiO2 nanocomposites exhibit highly efficient and stable visible light photocatalytic activity after Ag2C2O4 incorporation with P25-TiO2 (see Fig. 6A). This means that combination with P25-TiO2 helps to efficiently utilize the photo-corrosion of Ag2C2O4 thereby leading to greatly increased photocatalytic activity and stability of Ag2C2O4/TiO2 under visible light irradiation, which possibly because the surface plasmon resonance of Ag derived from the photo-corrosion of Ag2C2O4 as discussed in detail below. With increase the weight ratio of Ag2C2O4, the removal rate of propylene enhanced from 55% for Ag2C2O4/TiO2-0.02 to 91% for Ag2C2O4/TiO2-0.25 followed by decline to 74% for Ag2C2O4/TiO2-0.3, respectively. In the case of Ag2C2O4/TiO2-0.25, the best photocatalytic behavior can be ascribed to the optimized optical absorption and transient photocurrent response. Fig. 6B shows the change of CO2 production with irradiation time. It can be seen that the change trends of CO2 production over all Ag2C2O4/TiO2 composites are similar to that of pure Ag2C2O4. As light is turned on, the produced CO2 first sharply reaches the maximum, and then
gradually decreases to a steady value. Moreover, the amount of produced CO2 is related with not only the activity of catalyst but also the content of Ag2C2O4 in Ag2C2O4/TiO2 composites. This indicates that photodecomposition of Ag2C2O4 in Ag2C2O4/TiO2 composites also happened under visible light. The kinetic behavior of Ag2C2O4/TiO2 nanocomposites in degradation of propylene were measured and analyzed. Ag2C2O4/TiO2-0.25 photocatalyst was selected as representative sample to evaluate the photocatalytic activities under different flow rate of the feed gas. As the flow rate was adjusted to 554, 360, 300, 239 and 194.6 mL·h-1, respectively, the removal rate of propylene was tested to be 50%, 62%, 67%, 75% and 80% accordingly. Assuming that the propylene degradation follows the first-order kinetics, the equation ln [1/(1-x)] = kt will be applied, where x is the removal rate of propylene, k is the reaction rate constant and t is the irradiation time. In this photocatalytic reaction system (i.e. system I), dividing the irradiated photo-reactor volume (25 mL) by the flow rate of the feed gas to get the irradiation time t. For example, the reaction time t equals to 0.0451 h when the flow rate is 554 mL·h-1 (t = 25mL/554 mL·h-1). The curve of ln [1/ (1-x)] vs. irradiation time t is shown in Fig. 7. It can be seen that the value of linear correlation coefficient R for Ag2C2O4/TiO2-0.25 is 0.9984, confirming the first order trend of degradation of propylene by Ag2C2O4/TiO2 catalyst. Moreover, the reaction rate constant of Ag2C2O4/TiO2-0.25 was calculated to be 11.13 h-1. The results demonstrate that Ag2C2O4/TiO2-0.25 can be used as highly efficient photocatalyst for degradation of propylene under visible light. The degradations of acetaldehyde gas over Ag2C2O4/TiO2 nanocomposites, P25-TiO2 and Ag2C2O4 were also tested under visible light irradiation in system II. All of the degradation was carried out after the dark adsorption equilibrium. As shown in Fig. 8a. TiO2 exhibits nearly no
activity in this experiment. And the Ag2C2O4/TiO2 composite exhibits much higher activity than pure Ag2C2O4. The linear relationship between ln (C0/C) and irradiation time are shown in Fig. 8b. The reaction rate constant k and linear correlation coefficient R for Ag2C2O4/TiO2 composite with different weight ratio of Ag2C2O4 to TiO2 in degrading acetaldehyde gas are summarized in Table 1. The values of R for all the cases were higher than 0.97, confirming the first order nature of the degradation of acetaldehyde by Ag2C2O4/TiO2 composites. The rate constant k of Ag2C2O4/TiO2 composite is in the order of Ag2C2O4/TiO2-0.25 > Ag2C2O4/TiO2-0.08 > Ag2C2O4/TiO2-0.02 > Ag2C2O4/TiO2-0.3, indicating that, similar to the oxidation of propylene, Ag2C2O4/TiO2-0.25 catalyst still shows the highest activity for the degradation of acetaldehyde. As many literatures reported, silver-based semiconductor photocatalysts are easy to decrease their photocatalytic activity due to the photocorrosion. Yu and coworkers found that Ag2CO3 almost lose its activity in the second run of the recycling reaction for the degradation of methyl orange
19
. Fa and coworkers reported that the photocatalytic degradation ability of
Ag3PO4/Ag2CO3 photocatalyst continuously decreased with increase cycling runs possibly due to the decomposition of Ag3PO4 and Ag2CO3 into metallic silver
45
. Wang and coworkers revealed
that both AgBr/Ag3PO4 and AgBr/Ag3PO4/TiO2 showed apparent downtrend activity in successive MO degradation runs
46
. To compare the stability between pure Ag2C2O4 and
Ag2C2O4/TiO2-0.25 nanocomposite photocatalyst, recycle experiments of propylene degradation in association with CO2 production were carried out. As shown in Fig. 9a, it can be seen that the visible light photocatalytic activity over pure Ag2C2O4 slightly decreased from 52.7% to 40.6% after four cycles. In the first run, as discussed above, pure Ag2C2O4 experienced photo-corrosion in the process of propylene degradation and a CO2 concentration spike with the value of 34711
ppmV appeared as long as the light on, which immediately declined continuously until the light off. However, the CO2 concentration spike surprisingly disappeared from the second to fourth cycling run. Combined with above two results, it can be inferred that the photo-corrosion was almost complete in the first cycle and most of carbon dioxide detected resulted from the decomposition of Ag2C2O4. Furthermore, the CO2 concentration platform should be almost derived from the photodegradation of propylene with regard to the second to fourth cycling run. Contrary to pure Ag2C2O4, Ag2C2O4/TiO2-0.25 possessed highly stable visible light photocatalytic activity in four successive cycling runs (see Fig. 9c). However, similar to pure Ag2C2O4, a CO2 concentration spike with the value of 17531 ppmV also initially appeared as long as the light on and immediately declined continuously until the light off for Ag2C2O4/TiO2-0.25. It means that both pure Ag2C2O4 and Ag2C2O4/TiO2-0.25 nanocomposite underwent the same photo-corrosion process. To reveal the relation between the photo-decomposition of Ag2C2O4 and the activity of Ag2C2O4 as well as Ag2C2O4/TiO2, we further conducted SEM, TEM, XPS and XRD analysis of Ag2C2O4 and Ag2C2O4/TiO2-0.25 after visible light irradiation. It can be observed from Fig. 10a that an obvious hole inserted with several particles is present on a smooth surface of Ag2C2O4, and many raised circular particles are also distributed on the other surface. It is clear that pristine Ag2C2O4 underwent the photodecomposition introducing silver particles and CO2 gas during visible light irradiation. Fig. 10b is the TEM image of Ag2C2O4/TiO2-0.25 composite after visible light irradiation. There is no obvious change can be seen from the image comparison between before (Fig. 10c) and after irradiation. This may be resulted from the particle size of adhered Ag2C2O4 or Ag, which is too small to distinguish. However, from Ag Auger MVV spectra of
Ag2C2O4 (Fig. 11a) and Ag2C2O4/TiO2-0.25 (Fig. 11b) before and after irradiation, it is seen that the peak at 1131.2 eV largely shifted to 1128.6 eV after four-cycle propylene photodegradation experiments for both samples. This indicates that partial Ag+ in Ag2C2O4 and Ag2C2O4/TiO2-0.25 were reduced to Ag 26, confirming again that the photo-corrosion happened during photocatalytic process. Furthermore, Fig. 12a shows XRD patterns comparison of Ag2C2O4 before and after irradiation. It can be seen that, after recycle experiments, the typical characteristic peaks of Ag at 38.2◦, 44.2◦, 64.4◦, and 77.4◦ clearly appeared in addition to the diffraction peaks of Ag2C2O4 47, 48, which is well consistent with XPS results. It demonstrated that during photocatalytic process, wide-band-gap Ag2C2O4 semiconductor is gradually changed into Ag@Ag2C2O4 composites. Similarly, as for Ag2C2O4/TiO2-0.25, after successive four-cycle experiments, the intensity of characteristic diffraction peaks of Ag2C2O4 dramatically decrease and the cubic Ag is demonstrated to be formed with five intense peaks at 38.2◦, 44.2◦, 64.4◦, 77.4◦, and 80.7◦, which can be assigned to (111), (200), (220), (311) and (222) lattice planes. This, in combination with XPS results, indicates that Ag2C2O4/TiO2-0.25 has transformed into a new photocatalyst Ag@Ag2C2O4/TiO2 during visible light irradiation process. 3.6 Photocatalytic mechanism of Ag2C2O4 and Ag2C2O4/TiO2 under visible light irradiation. It seems impossible for Ag2C2O4 or Ag2C2O4/TiO2 to show visible light activity due to its wide band gap. Surprisingly, the photo-corrosion of Ag2C2O4 happened during photocatalytic process affording
nascent
metallic
Ag
and
a
plasmonic
photocatalyst
Ag@Ag2C2O4
or
Ag@Ag2C2O4/TiO2 formed thereby. It can be expected that the nascent metallic Ag scattered in situ on the substrate Ag2C2O4 or P25-TiO2. As we all know, the plasma resonance effect of noble metals such as Au and Ag can be used to promote the photocatalytic performance
49
. Namely, on
the one hand, if semiconductor can be excited to generate e- - h+ pair by visible light, Ag will effectively trap the photogenerated e- and transfer to adsorbed oxygen to produce reactive oxygen species O2- radicals. Kuai and coworkers successfully fabricate a stable and highly efficient direct sunlight plasmonic photocatalyst Ag-AgBr through a facile hydrothermal 50. It was found that the produced Ag nanoparticles not only prevent the decomposition of AgBr but also suppress the recombination of e- and h+. On the other hand, silver nanoparticles can also be excited by visible light to generate e- and h+ due to the plasma resonance effect. Many studies have reported that the excited electrons from noble metals, such as Au and Ag, can also be injected into the conduction band (CB) of TiO2 to react with adsorbed oxygen forming superoxide radicals
51-53
. The visible
light photocatalytic mechanisms over Ag2C2O4 and Ag2C2O4/TiO2 are proposed. As shown in Fig. 13a, the plasma-excited holes from the surface of Ag could oxide the organics into CO2
50, 54, 55
and the plasma-excited electrons could transfer to the conduction band of Ag2C2O4 because of the more positive CB energy of Ag2C2O4 (-0.11 eV) than that of TiO2 (-0.50 eV). However, the electrons transferred to CB of Ag2C2O4 cannot react with adsorbed O2 due to the lower potential of CB (-0.11) than that of O2/O2- (-0.33)
56
. As for Ag@Ag2C2O4/TiO2 (Fig. 13b) evolved from
Ag2C2O4/TiO2, besides the excited h+ for the oxidation of organics, the excited electrons transferred to the CB of TiO2 can be applied to produce reactive oxide radicles O2- due to the more negative potential (-0.50 eV) than that of O2/O2- (-0.33). Two degradation paths make the visible-light-activity of Ag2C2O4/TiO2 to be much higher than that of Ag2C2O4. Furthermore, the size and loading amount of the silver nanoparticles strongly affect the photocatalytic performance of the plasmonic photocatalyst Ag@Ag2C2O4/TiO2. Tian and coworkers have reported that interfacial electron transfer process depends on the size of noble metals on TiO2 surface 57. When
the size of gold particles is too large, the Fermi energy level of gold particles will be lower than the conduction band edge of TiO2 and hence the generated electrons from TiO2 can be captured by Au particles. However, for quantum sized gold particles, the excited electrons can transfer to the conduction band of TiO2 from gold particles since its Fermi energy level is higher than TiO2 CB edge. In the present work, the size of Ag particles of Ag@Ag2C2O4/TiO2 (see Fig. 10b) is about 1 ~ 5 nm, which is much smaller than that of Ag particles on the surface of Ag2C2O4 (see Fig. 10a). This may be another factor that Ag2C2O4/TiO2 exhibited more superior photocatalytic performance than that of Ag2C2O4. 4. Conclusions In summary, pure Ag2C2O4 and Ag2C2O4/TiO2 nanocomposite photocatalysts have been synthesized by a facial liquid precipitation method. The visible-light activity and the kinetic behavior of Ag2C2O4 and Ag2C2O4/TiO2 have also been evaluated by the degradation of propylene and acetaldehyde gas. Compared to pure Ag2C2O4 and P25-TiO2, Ag2C2O4/TiO2 exhibited much higher visible light activity and its kinetic behavior was accordant with the first order reaction formula for the degradation of propylene and acetaldehyde. The comparison of SEM, TEM, XPS and XRD results between before and after visible light photocatalytic reaction demonstrated that both Ag2C2O4 and Ag2C2O4/TiO2 experienced the photo-decomposition generating nascent metallic Ag during photocatalytic process. The nascent metallic Ag was expected to scatter in situ on the substrate Ag2C2O4 or P25-TiO2, forming the plasmonic photocatalyst Ag@Ag2C2O4 or Ag@Ag2C2O4/TiO2. The mechanism of highly efficient and stable photocatalytic activity was proposed. On the one hand, Ag2C2O4 undergoes firstly photo-corrosion during the initial photocatalytic process generating nascent metallic Ag on the surface of Ag2C2O4, which
subsequently endowing the Ag@Ag2C2O4 with visible light phtocatalytic activity due to the plasma resonance of Ag. On the other hand, Ag2C2O4/TiO2 nanocomposites also experience the same journey resulting in the formation of Ag@Ag2C2O4/TiO2. Differently from Ag@Ag2C2O4, two paths are involved to separate the electron and hole excited by the photon of nascent metallic Ag so as to acquire much higher phtocatalytic activity for Ag@Ag2C2O4/TiO2. Acknowledgement The authors acknowledge the financial support provided by the Science and Technology Research Project of Henan province (Grant No. 15A150040, 162102210178).
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Figure Captions Fig. 1. SEM image (a) of Ag2C2O4 and representative TEM images of P25-TiO2 (b) and Ag2C2O4/TiO2-0.25 (c) before irradiation. Fig. 2. XRD patterns of different photocatalysts: (a) Ag2C2O4/TiO2-0.02, (b) Ag2C2O4/TiO2-0.08, (c) Ag2C2O4/TiO2-0.15, (d) Ag2C2O4/TiO2-0.25, (e) Ag2C2O4/TiO2-0.3, (f) Ag2C2O4. Fig. 3. High-resolution XPS spectra of C 1s (A) and Ag 3d (B) for Ag2C2O4 (a) and Ag2C2O4/TiO2-0.25 (b). Fig. 4. UV-vis diffuse reflectance spectra of Ag2C2O4, P25-TiO2 and Ag2C2O4/TiO2 composite samples: (a) Ag2C2O4/TiO2-0.02,
(b)
Ag2C2O4/TiO2-0.08,
(c)
Ag2C2O4/TiO2-0.15,
(d)
Ag2C2O4/TiO2-0.25,
(e)
Ag2C2O4/TiO2-0.3, (f) Ag2C2O4, (g) P25 TiO2. Inset: change of visible light absorptivity at 500 nm with the ratio of Ag2C2O4 to TiO2. Fig. 5 Transient photocurrent responses of different electrodes under visible light irradiation: (a) Ag2C2O4/TiO2-0.02,
(b)
Ag2C2O4/TiO2-0.08,
(c)
Ag2C2O4/TiO2-0.15,
(d)
Ag2C2O4/TiO2-0.25,
(e)
Ag2C2O4/TiO2-0.3,(f) P25-TiO2. Fig. 6. Visible light photoactivity of P25, Ag2C2O4 and Ag2C2O4/TiO2 composites evaluated by C3H6 removal (vol%) (A); Change of CO2 production with irradiation time (B). (a) Ag2C2O4/TiO2-0.02, (b) Ag2C2O4/TiO2-0.08, (c) Ag2C2O4/TiO2-0.15, (d) Ag2C2O4/TiO2-0.25, (e) Ag2C2O4/TiO2-0.3, (f) Ag2C2O4, (g) P25-TiO2. Fig. 7. The first-order kinetic for degradation of propylene by Ag2C2O4/TiO2-0.25 under visible light irradiation. Fig. 8. Degradation of acetaldehyde (a) and the first order kinetic (b) over different catalyst under visible light irradiation. Fig. 9. Repetitive degradations of propylene over pure Ag2C2O4 (a) and Ag2C2O4/TiO2-0.25 composite (c) as well as recycling change of CO2 production with irradiation time over Ag2C2O4 (b) and Ag2C2O4/TiO2-0.25 (d) under visible light irradiation. Fig. 10. SEM image of Ag2C2O4 after irradiation (a) as well as TEM image of Ag2C2O4/TiO2-0.25 after (b) and before (c) irradiation. Fig. 11. Auger Ag MVV XPS spectra of as-prepared Ag2C2O4 (a) and Ag2C2O4/TiO2-0.25 (b) before and after visible light irradiation. Fig. 12. XRD patterns of as-prepared Ag2C2O4 (a) and Ag2C2O4/TiO2-0.25 (b) before and after visible light irradiation. Fig. 13. Photocatalytic mechanism diagrams of Ag2C2O4 (a) and Ag2C2O4/TiO2-0.25 (b). Table 1.The reaction rate constant k and correlation coefficient R for Ag2C2O4/TiO2 composite with different weight ratio of Ag2C2O4 to TiO2 in degrading acetaldehyde test under visible light irradiation.
Figure 1. SEM image (a) of Ag2C2O4 and representative TEM images of P25-TiO2 (b) and Ag2C2O4/TiO2-0.25 (c) before irradiation.
Figure
2.
XRD
patterns
of
different
photocatalysts:
(a)
Ag2C2O4/TiO2-0.02,
(b)
Ag2C2O4/TiO2-0.08, (c) Ag2C2O4/TiO2-0.15, (d) Ag2C2O4/TiO2-0.25, (e) Ag2C2O4/TiO2-0.3, (f) Ag2C2O4.
Figure 3. High-resolution XPS spectra of C 1s (A) and Ag 3d (B) for Ag2C2O4 (a) and Ag2C2O4/TiO2-0.25 (b).
Figure 4.
UV-vis diffuse reflectance spectra of Ag2C2O4, P25-TiO2 and Ag2C2O4/TiO2
composite samples: (a) Ag2C2O4/TiO2-0.02, (b) Ag2C2O4/TiO2-0.08, (c) Ag2C2O4/TiO2-0.15, (d) Ag2C2O4/TiO2-0.25, (e) Ag2C2O4/TiO2-0.3, (f) Ag2C2O4, (g) P25 TiO2. Inset: change of visible light absorptivity at 500 nm with the ratio of Ag2C2O4 to TiO2.
Figure 5 Transient photocurrent responses of different electrodes under visible light irradiation: (a) Ag2C2O4/TiO2-0.02, (b) Ag2C2O4/TiO2-0.08, (c) Ag2C2O4/TiO2-0.15, (d) Ag2C2O4/TiO2-0.25, (e) Ag2C2O4/TiO2-0.3,(f) P25-TiO2.
Figure 6. Visible light photoactivity of P25, Ag2C2O4 and Ag2C2O4/TiO2 composites evaluated by C3H6 removal (vol%) (A); Change of CO2 production with irradiation time (B). (a) Ag2C2O4/TiO2-0.02, (b) Ag2C2O4/TiO2-0.08, (c) Ag2C2O4/TiO2-0.15, (d) Ag2C2O4/TiO2-0.25, (e) Ag2C2O4/TiO2-0.3, (f) Ag2C2O4, (g) P25-TiO2.
Figure 7. The first-order kinetic for degradation of propylene by Ag2C2O4/TiO2-0.25 under visible light irradiation.
Figure 8. Degradation of acetaldehyde (a) and the first order kinetic (b) over different catalyst under visible light irradiation.
Figure 9. Repetitive degradations of propylene over pure Ag2C2O4 (a) and Ag2C2O4/TiO2-0.25 composite (c) as well as recycling change of CO2 production with irradiation time over Ag2C2O4 (b) and Ag2C2O4/TiO2-0.25 (d) under visible light irradiation.
Figure 10. SEM image of Ag2C2O4 after irradiation (a) as well as TEM image of Ag2C2O4/TiO2-0.25 after (b) and before (c) irradiation.
Figure 11. Auger Ag MVV XPS spectra of as-prepared Ag2C2O4 (a) and Ag2C2O4/TiO2-0.25 (b) before and after visible light irradiation.
Figure 12. XRD patterns of as-prepared Ag2C2O4 (a) and Ag2C2O4/TiO2-0.25 (b) before and after visible light irradiation.
Figure 13. Photocatalytic mechanism diagrams of Ag2C2O4 (a) and Ag2C2O4/TiO2-0.25 (b)
Table 1 The reaction rate constant k and correlation coefficient R for Ag2C2O4/TiO2 composite with different weight ratio of Ag2C2O4 to TiO2 in degrading acetaldehyde test under visible light irradiation. Catalyst
Weight ratio of Ag2C2O4 to TiO2
Correlation coefficient R
Reaction rate constant k (min-1) for the degradation of acetaldehyde
Ag2C2O4/TiO2-0.02
0.02:1
0.983
0.00735
Ag2C2O4/TiO2-0.08
0.08:1
0.989
0.00764
Ag2C2O4/TiO2-0.25
0.25:1
0.996
0.00896
Ag2C2O4/TiO2-0.3
0.30:1
0.979
0.00719